Is my battery dying, or is this something else? See details.

I am trying to determine if I actually have a dying battery, or if my phone is confused about the battery status or if there's some sort of charging problem. The phone is a little over two years old.

In the last few days, I woke up to a phone with a drained battery after leaving it well charged the night before. It turned itself off.

I usually charge it to 80-90% and try not to let it go below 30%. Yesterday, it would charge somewhat normally, but a little slower, then drain very quickly. I uninstalled one app that took a lot of the battery even though I wasn’t using it, but that didn’t help. I charged it on three different chargers and power sources during the day (twice on A/C with different chargers, and once over USB). It kept draining quickly.

Before going to sleep, I charged it to 100% and left it on. In the morning it was 91% and said I had 4 days left, which was what it used to do.

I worked for about 15 minutes on backing up my data, and then it said it had only 8 hours left. While I watched it, it jumped to 7 hours left. While typing this, it’s down to 6 hours left, 80%.

All the functions work normally.

So, is it the battery, or something in the software? Or something else?

The original post was mine (I'm now signed in) and I want to add that it's a Moto G 3rd generation. Carrier is AT&T.

The phone has calculated new projected hours of battery left, and it's increased, but overall it's still discharging fairly quickly even though almost nothing is running. I exchanged a few text messages, took one photo, and was on wifi for less than a minute or two, and it's down to 53% -- early afternoon. Was at 91% at around 7 this morning. The usage data shows mostly screen and standby. The screen is set fairly dark, and I manually lock it when not in use.